Ultrasound equipment revolutionizes predictive maintenance

It now provides improved monitoring of processes and equipment in wide ranging applications.

By Leonard Bond, Paul Panetta, Richard Pappas and Judith Bamberger

It's estimated that more than $1 trillion is spent each year to replace perfectly good equipment because no reliable and cost-effective method is available to predict its remaining life.

Ultrasonic techniques offer the ability to acquire physical property data that indicates process quality or equipment problems. Ultrasonic signals are inherently well suited to provide information about materials moving through piping or vessels. While ultrasonic sensors are used widely to measure level and flow, they also can monitor fluid and slurry velocity and rheology, particle size distribution and concentration, and can detecting plugged filters and pipeline fouling without a need for sampling manually or shutting down a process.

The physics of ultrasonics are not new. Ultrasonic measurements have been available for more than 50 years, but until recently, only as highly specialized and costly scientific instruments. With advances in computers and microelectronics, ultrasonic systems can be a cost-effective and valuable addition to a company's process measurement capabilities. High-speed analog-to-digital conversion cards now allow measurements to be conducted on-line and in real-time, often preventing process downtime.

In addition, ultrasound can penetrate vessel and process walls to interrogate fluids and dense, opaque suspensions. Noisy process conditions won't degrade the performance of the measurement electronics because the ultrasonic signal frequencies differ from those of machinery.

Researchers at the Department of Energy's Pacific Northwest National Laboratory in Richland, Wash. are developing a suite of complementary instruments and methodologies to monitor physical characteristics of flowing materials, mixtures or chemical reactions quickly, accurately and non-invasively.

A harsh beginning
The researchers initially focused their ultrasonic measurement expertise on what is arguably the toughest measurement challenge--the highly toxic and rugged environment inside radioactive waste tanks.

To monitor colloids, slurries and particulate-laden flows contained within these million-gallon tanks, it was necessary to develop innovative means to characterize the material remotely, yet get information that was deeper than just near the surface. Ultrasonics proved to be a valuable method.

The research team now is applying its know-how to food processing, polymer process streams, consumer product manufacturing, chemicals, petrochemicals, pharmaceuticals and coatings.

Measuring the flow
The ultrasonic Doppler velocimeter provides non-invasive, real-time capabilities for continuous monitoring of key physical properties of liquids and slurries. Initially created to monitor the flow of waste streams and identify potential plugging problems, the technology has shown itself to be even more versatile, with an array of possible applications in manufacturing.

The design has been refined recently to address limitations in the maximum measurable flow rate, the velocity profile accuracy and the Doppler signal-to noise ratio. It integrates interface detection and measurements of concentration and rheology into a single ultrasonic monitoring unit.

Known as the Real-Time Ultrasonic Rheometer and Fluid Characterization Device, this technology represents a combination of advances in signal processing, sensors and miniaturized electronics. It relies on measurement of the Doppler frequency shift of scatters within a pipe flow or moving tracer particles. Changes in the velocity profile provide early detection of flow problems caused by fluctuations in the slurry's rheology and physical properties. They indicate the presence of air or the onset of settling, saltation and gelation.

Rugged enough for manufacturing environments, the device provides novel performance in terms of automation, resolution, penetration and maximum flow rates. It economically monitors and extracts information, including shear thinning, yield stress and the build-up of wall deposits.

Because ultrasound waves penetrate metal and plastic pipes and vessels, transducers can be mounted into a spool piece or simply strapped to the outside of the piping. With its multi-transducer configuration, it continuously sends and receives signal pulses through the piping and flowing material and extracts ultrasonic time-of-flight information, amplitude measurements and range-gated Doppler data. This information is combined with a temperature measurement to yield the speed of sound in the material, ultrasonic attenuation, average flow rate and flow velocity profiles.

Ultimately, the information extracted is analyzed, used to measure properties, such as texture and consistency, and displayed on a computer for use by operators.

The researchers field tested the technology at a tomato processing plant and at a personal care product manufacturer--two operations that require exacting texture and consistency control. In addition, the device can monitor polymer process streams.

Early detection of fouling, clogging and failure
Ultrasound has also proved beneficial in early detection of fouling and clogging in filters and piping. Current methods that use backflow and permeate flux measurements do identify fouling, but rather late in the process. Plant managers can save money and reduce unnecessary downtime by using ultrasonic measurements to identify fouling before it becomes a problem and to determine the best time to backflush or change out equipment.

Pacific Northwest National Laboratory used the technology to detect fouling on reverse osmosis units at a pilot scale service water system, a polymer processing plant and at a petrochemical facility. It likely has applications at a variety of industrial sites.

One researcher developed pulse-echo ultrasonic measurement into a technique called acoustic time domain reflectrometery (ATDR). It can be used to study fouling and compaction in membrane separation systems. Other applications include monitoring sedimentation, solidification, membrane treatments and characterization of inorganic, bio and particulate fouling in a variety of processes.

ATDR detects interfaces and growth of settled solids layers. These include both symmetric and asymmetric anomalies. Symmetrical interfaces include the presence or growth of scale on the pipe wall, or the development of gelation or a plug in the core of the pipe. Asymmetric interfaces include a bed of settled solids or saltation flow at the bottom of a pipe or the presence of an air bubble at the top of a pipe. The time-of-flight associated with the reflected ultrasonic signal detects these interfaces. ATDR also is used for predictive engineering or diagnostics when assessing the status of filters and other process equipment.

Ultrasonic measurements are being tested as part of an on-line, intelligent Self-Diagnostic Monitoring System (SDMS) for the Nuclear Energy Research Initiative (NERI). SDMS data analysis, including ultrasonic data, obtained from a test-bed service water operation extends the unique capabilities of the diagnostic methodology found in the Decision Support for Operations and Maintenance (DESOM). Diagnostic capabilities can now detect emerging conditions that, if allowed to continue, could lead to component inefficiencies or failure.

Particle size distribution and concentration
When ultrasound passes through a slurry, the interaction of sound with slurry particles reduces the signal strength. Analyzing signal attenuation provides real-time, in-situ measurement of slurry concentration and particle size distribution. This was demonstrated during the development of techniques to remediate hazardous waste tanks. The ability to make these measurements without having to sample the slurry has obvious benefits.

As the particle size and acoustic frequency are changed, different effects dominate the relative importance of attenuation. Tracking the three regimes and the transitions between them characterize distribution changes in real time. For larger particles, the transition tends to occur at lower frequencies: for smaller particles it occurs at higher frequencies. This information has been used to develop a sensor that reports actual process conditions in stirred tanks, pipelines or other process vessels.

Into the thicket
Now, researchers are moving beyond attenuation and velocity measurements to a novel approach to monitoring particle size and concentration in dense slurries. Now in early stages of development, the new approach uses diffuse field and backscattering techniques to enhance real-time, in-line monitoring. The work is focused on radioactive waste tank characterization, but the research will benefit many processing environments.

In the paint industry, for example, the technique can ensure that particle concentrations optimize the paint's aesthetic and protective performance. In addition, the technique could help pharmaceutical companies and others monitor fermentation processes, including mixing and stratification, which are key to the effective development of medicines, beverages and other products. Other applications include food processing, chemical processing, chemical polishing compounds and ceramics.

From lab to plant floor
Because much of the science underlying these cutting-edge ultrasonic techniques has been developed with government funding, the Pacific Northwest National Laboratory is actively seeking to transfer the technology to industry. Laboratory researchers are currently working with several companies to advance the technologies. While there are currently no commercially available devices, the laboratory is happy to work with companies to customize applications or license technologies for exclusive or non-exclusive applications.

Judith Bamberger, Leonard Bond, Paul Panetta and Richard Pappas are staff engineers and scientists at the Pacific Northwest National Laboratory, Richland, Wash.